A backlight display has become widespread in which a liquid crystal layer is illuminated from the backside to emit light. On the underside of the liquid crystal layer, a backlight unit is provided. The backlight unit is generally equipped with a bar-shaped lamp as a light source and a plurality of laminated optical sheets. Each of these optical sheets has specific optical characteristics such as refraction and diffusion. Specific examples of the optical sheet include: a square plate-shaped light guide plate whose end portion is arranged along a lamp; a light diffusion sheet provided on the surface of the light guide plate; a light-collecting sheet such as a prism sheet which is arranged on the surface of the diffusion sheet. Another type of the backlight unit is, for example, of a direct backlight type in which a lamp is located on the non-display side of a display and optical sheets are located between the lamp and the display.
A sheet (Patent Literature 1) has been recently proposed which has both functions of a diffusion sheet and a function of a light-collecting sheet, that is, has an enhanced changing a direction function of light without decreasing the amount of light emission in the normal direction. When such a sheet is employed, there are effects that the brightness of the display is increased and that the display is made thinner by reducing the number of the optical sheets in a backlight unit and by reducing the distance between the lamp and the display.
Such a sheet is proposed to be manufactured by a method in which two kinds of synthetic resins are multilayered in the width direction by a mixer. It is, however, not possible to obtain a sheet which has a large area and a uniform performance by this method. This is because disappearance of a layer, unification of layers or the like is caused by a considerable flow disturbance due to repeated deformations by a mixer or a deformation through an extrusion die when a very large number of layers each having a minute width are to be obtained by the mixer method.
Such a method of laminating sheets in the width direction by using a mixer is also disclosed, for example, in Patent Literature 2. Even in cases where the number of layers is small, deformation of the layers is inevitable, as shown in FIG. 1 and FIG. 2 of Patent Literature 2.
In addition, a method is proposed in which an optical interconnection is obtained by using a complex device having a large number of slits (Patent Literature 3). In this method, a width-direction multilayer laminated film with a higher precision compared with a film obtained by a method in which a mixer is employed, can be obtained; however, the upper limit of the substantial number of the layers aligned in a line is 301, the shape, position and cross section area of the core layer is hard to be stable due to the configuration of the die and therefore, and it was difficult to obtain a width direction multilayer laminated film having a large area.
Patent Literature 1: JP 2001-91708 A
Patent Literature 2: JP 51-33177 A
Patent Literature 3: JP 2006-221145 A